Leachate, an aqueous solution created by the passage of fluids through waste piles, is a principal environmental concern. Since passage of the Clean Water Act, waste containment systems must be engineered to prevent migration of leachate into the groundwater underlying landfill sites. Conventionally, the containment and flow control of such leachate has been achieved by the use of one or more of compacted clay liners, various types of synthetic geomembranes, and synthetic clay liners.
It is well established that leachate can cause distress and damage to synthetic liner systems, causing leaks, and thereby polluting groundwater and the local environment. Therefore, the effective engineering and design of a containment system for a landfill or other similar structure requires drainage systems to be constructed above geomembrane liners which are disposed to remove these fluids. In fact, the USEPA regulatory guidance states that no more then a one-foot liquid head is allowable above a geomembrane in such an installation. In some conventional drainage systems, engineers specify that stone of uniform gradation be utilized as the leachate drainage layer at the base of a landfill. Stones are often specified to obtain a certain “diameter” and are measured in sieves that have specific diameters. This is because spheres touch at points of tangentiality. In some other drainage systems, engineers specify that processed tire chip aggregate of uniform sizes be utilized as the leachate drainage layer at the base of a landfill. Engineers skilled in the art of landfill design utilize the principle of tangentiality and require aggregate producers to manufacture stone particles that are relatively spherical. They achieve this by specifying uniform gradations of stone. A gradation refers to the distribution of stones with different “diameters.”
Thus, leachate collection systems are highly engineered layered structures and require engineered materials that are selected based upon factors such as their density, particle or aggregate size, compressibility, chemical compatibility, and other engineering parameters of the soil, stone and aggregate-based products.
Stone is highly non-compressible. Therefore, even when stones are subjected to compressive forces, voids exist in those spaces where the stones do not touch. Therefore, even under significant loading conditions, void spaces, or porosity may be obtained. The more open void space volume created, the greater the porosity. Typically, the more porous an installation or layer, the higher the resulting cost. For example, a stone with an effective size of ¼″ and a coefficient of uniformity of 2.5, typically costs much more then sand. This type of gradation is often classified as AASHTO 57 and is often utilized to create open-graded base course in landfills, roadways, and other installations needing a specified drainage capacity. Aggregate classifications are standardized for FHWA and DOT Transportation applications. In contrast, this degree of classification typically does not exist for environmental markets. For example, while a landfill in California may specify a stone of uniform gradation of average ½″, such specifications may not refer to the stone as an AASHTO 57 stone. This is so even though the transportation department or company that constructed the road to the landfill may have utilized the same exact stone and classified it as AASHTO 57. AASHTO 57 is often used as an open-graded base course (OGBC).
An open graded base course (OGBC) can be utilized as a means to convey fluids to leachate collection laterals and pipes. Still, in other systems engineers will specify sand as a natural material that offers both vertical permeability and horizontal transmissivity. As one skilled in the art of landfill design can appreciate, not all landfills require sand nor do they all require stone. Therefore, design of particular landfills is often site-specific. For example, engineers may require a stone drainage layer to achieve the regulatory requirements but the local geological conditions do not offer stone. When this occurs, contractors are required to purchase stone and have it transported over long distances. Such transportation costs significantly drives up the cost of construction of the landfill. In fact, engineers and other design personnel who procure construction aggregates typically estimate that the cost of aggregate supply doubles for every 25 miles of transport distance to the landfill site.
In conventional landfill construction, an OGBC may be placed to form a leachate collection system. These OGBC systems are typically used above primary geomembranes. Leachate collection systems are highly engineered layered structures and require engineered materials that are selected based upon factors such as their density, particle or aggregate size, compressibility, chemical compatibility, or other engineering parameters of the soil, stone and aggregate-based products.
Other engineering parameters reflect the importance of sufficient drainage in landfills. In fact, bioreactors and/or leachate recalculation facilities require high flowing materials. For example, theses types of structures collect all leachate and recirculate the fluids to help further consolidate the waste mass. This re-circulation results in increased void or air-space which results in more capacity and, consequently, more potential revenue for a site. Thus, the rate at which leachate and other fluids are transported away from the various layers of a landfill is a critical element in its useful life. Leakage rates that are excessive require the landfill to be closed and the leak to be corrected. Thus, inadequate drainage can be an extremely serious and costly problem affecting a landfill.
In one conventional method of approaching these drainage problems, an OGBC drainable layer formed of natural stone and aggregate materials is included above or beneath a geomembrane in an attempt to positively control fluids and dissipate pore pressures which commonly accumulate within these structures. Typically, an OGBC-drainable permeable layer also utilizes a geotextile for membrane protection and/or filtration. An OGBC is intended to be a porous drainage media that is capable of receiving fluids from the points of entry and then transporting them to designated discharge points in a timely manner. These systems often utilize AASHTO 57 stone. According to the FHWA, an AASHTO 57 stone has a permeability of 6,800 linear feet per day and any OGBC drainage layer should have a minimum permeability of 1,000 linear feet per day.
An OGBC is typically produced from stone that has been mined from quarries. A main distinguishing characteristic of OGBC materials is that they are usually delivered to work sites having a fairly uniform gradation per the specifications of the project engineer. Typically, project engineers use published standards for OGBC available from AASHTO, the Federal Highway Administration, or their resident state's department of transportation. Theoretically, the uniform gradation of OGBC materials typically creates voids of desired and predictable dimension between the pieces of stone when they are in place. Thus, desired flow rates through both vertical and horizontal planes of the OGBC can be increased or decreased somewhat predictably by selecting appropriate size distributions of the stone particulate material.
An OGBC can be costly to install and maintain, and can be difficult to control and predict with respect to quality. Although such gradations of stone typically create interconnecting void spaces or holes among and between the aggregate useful to facilitate the reception and transmission of fluid, an OGBC can take up a considerable volume of valuable space of the installation. An additional problem relates to the longevity of the chosen stone. Stone is made of different minerals, some of which minerals are soluble in water or in the harsh chemical environments which often exist in landfills. In fact, in Kentucky, certain OGBC leachate collection systems constructed of limestone have completely dissolved because of the chemical nature of the fluids passing through them.
Other disadvantages of OGBC's pertain to the additional elements that are required in an OGBC installation. Typically, a well graded granular or geotextile filter layer is needed above the OGBC in order to prevent contamination of the OGBC from the migration of fines. This extra filter layer further increases the construction costs of the landfill. Yet another problem with the use of OGBC's is that aggregate of sufficient quality is not always available or, if available, it's cost is uneconomical or prohibitively high. There is therefore a need for landfills and for landfill drainage systems that utilize components which can be engineered and manufactured offsite, and easily transported to the site and integrated economically into the landfill or other large structure, and to provide equivalent or superior flow to that of a conventional OGBC. There is a similar need for drainage elements suitable for integration into landfills and other large structures which take up much less space than conventional OGBC's.
The present geosynthetic drainage elements offer a solution to these problems. In general, geosynthetics are manufactured from polymeric materials, typically by extrusion, as substantially planar, sheet-like, or cuspidated products. Geosynthetics are usually made in large scale, e.g., several meters in width and many meters in length, so that they are easily adaptable to large-scale construction and landscaping uses. Many geosynthetics are formed to initially have a substantially planar configuration. Some geosynthetics, even though they are initially planar, are flexible or fabric-like and therefore conform easily to uneven or rolling surfaces. Some geosynthetics are manufactured to be less flexible, but to possess great tensile strength and resistance to stretching or great resistance to compression. Certain types of geosynthetic materials are used to reinforce large manmade structures, particularly those made of earthen materials such as gravel, sand and soil. In such uses, one purpose of the geosynthetic is to hold the earthen components together by providing a latticework or meshwork whose elements have a high resistance to stretching. By positioning a particular geosynthetic integral to gravel, sand and soil, that is with the gravel, sand and soil resident within the interstices of the geosynthetic, unwanted movement of the earthen components is minimized or eliminated.
Most geosynthetic materials, whether of the latticework type or of the fabric type, allow water to pass through them to some extent and thus into or through the material within which the geosynthetic is integrally positioned. Thus, geosynthetic materials and related geotechnical engineering materials are used as integral parts of manmade structures or systems in order to stabilize their salient dimensions.
Before the present invention, the only geosynthetic materials available for landfill drainage were exclusively limited to drains at the edge or shoulder of a landfill. These edge-drain systems are commonly located within a covered trench originally dug along the shoulder of the landfill. Conventional edge drain geosynthetics, however, cannot withstand the repeated dynamic loads that are present directly beneath heavy overburdens, such as those typically found in land fills and other large structures. Geosynthetic drainage materials have been utilized also on side slopes of landfills in order to ameliorate stability difficulties associated with construction of granular material drains. Geosynthetic drainage materials of dimensions up to 275 mils thick have been utilized to complement sand or to substitute for sand as a natural material at the floor of landfill. However, such geosynthetic products have never been engineered to achieve flow rates and void-maintaining capabilities sufficient to replace stone. The present invention relates generally to synthetic void-maintaining structures with high permittivity and high transmissivity that are capable of partially or fully replacing stone in landfills and other large structures by maintaining voids of sufficient dimensions to permit the timely egress of undesirable fluids.
The present invention provides a series of Void-Maintaining Synthetic Drainable Base Courses (“VMSDBC's”) of polymeric material, and related methods, for designing and constructing leachate collection systems and drainage systems. The present VMSDBC's and methods thereby eliminate or minimize the amount of conventional open-graded stone that might otherwise be required. Until the present invention, no geosynthetic material had been designed or implemented that could provide a drainage system of equivalent or superior drainage to those of an OGBC as utilized to convey fluids in a conventional landfill. Similarly, until the present invention, no geosynthetic material had ever been designed that could maintain voids of defined and sufficient dimensions while undergoing the repeated dynamic cycles of fluid infiltration and exposure demanded of bioreactors and re-circulation facilities.
The present VMSDBC void-maintaining system is the first such synthetic material that allows those skilled in the art of landfill design to replace stone. Water migrates and enters the VMSDBC system and then travels through the VMSDBC to locations or areas where the fluid is then conveyed for discharge in a timely manner in designated areas of a landfill, or outside of it. The present invention thus offers a synthetic product that overcomes the many deficiencies of the conventional OGBC.
Thus, the present invention relates generally to synthetic void-maintaining structures with high permittivity and high transmissivity that are capable of extending the life of a landfill. The present invention thus overcomes stability concerns of other geosynthetics which are not truly suitable for use as void-maintaining drainage structures in landfills and other large structures. Numerous embodiments of the present VMSDBC and methods overcome the disadvantages of the conventional OGBC systems by providing a plurality of interconnected voids of great mechanical and dimensional stability while simultaneously providing sufficient horizontal flow to perform in accordance with “Good to Excellent” drainage performance when assessed with respect to AASHTO definitions. These performance attributes are unique to the present VMSDBC drainage elements and landfills, which eliminate many of the problems associated with fluids underlying large structures that are not resolved by conventional OGBC systems or any conventional geosynthetic product. By eliminating these problems, VMSDBC's of the present invention extend the useful life of the landfill by increasing the effective amount of airspace.
In accordance with other aspects of the present invention, the VMSDBC's of the invention can be positioned in a landfill to maximize their effectiveness. For example, a VMSDBC can be positioned directly above a geomembrane or beneath a geomembrane. Moreover, a VMSDBC of the invention can be made in large pieces, for example, in pieces several meters wide and many meters long. For convenience and installation, however, a VMSDBC and its components may be installed in portions which are interconnected such that the interconnecting voids are of sufficient dimension that the leachate can move freely through the SDBC and be connected to drain means such as a perforated pipe, drainage ditch, or culvert adjacent to the landfill.
In an important aspect, VMSDBC's of the invention, maintain the preferred void dimensions even under substantial loads. For example, typically the lower surface of the super stratum, that is, the upper fluid-transmissible layer, and the upper surface of the substratum, that is, the lower fluid-transmissible layer, are prevented from having contact with one another when the upper surface of the substratum and the lower surface of the super stratum are placed under sustained loads above 10,000 psf and the lower surface of the substratum and upper surface of the super stratum are in contact with a soil environment for a duration of not less than 100 hours.
Other advantages of the present VMSDBC's can be seen with respect to their fluid-transmitting capacity. For example, in some embodiments, a VMSDBC of the present invention typically exhibits a fluid transmitting capacity of at least 4,000 ft.3/day/ft when tested under a normal load of 15,000 psf, and at a gradient of 2% per ASTM D 4716. Thus, the present VMSDBC's exhibit superior fluid-transmitting characteristics and meet the specifications for classification as “Excellent to Good” performance under AASHTO's definitions.
Advantageously, a VMSDBC according to the present invention is superior to conventional drainage elements, inter alia, because it is capable of resisting long-term compressive stress to the extent that it resists creep deformation and structural catastrophic collapse under load by retaining 60% of its external dimensional thickness after 10,000 hours under a sustained normal load of 10,000 pounds per square foot. Preferably, a VMSDBC according to the invention, comprises an upper fluid-transmissible surface, and the core is pervious to the vertical migration of fluids. Furthermore SDBCs are preferably constructed and arranged to transmit fluids to discharge points within or at the perimeter of a landfill whereby the piping or other collection means is designed to receive fluids transported from within the landfill by means of the SDBC.
Void-maintaining synthetic drainable base courses (“VMSDBC's”) of the present invention can be fabricated into panels of various lengths and widths by using conventional means to weld, adhere, tie or sew SDBC sections to one another to form a continuous SDBC underneath construction soils, landfill materials, or waste.